Marit Allen, who has died aged 66 of a brain aneurism, pulled off a powerful use of movie-costume-as-character in the scene in Brokeback Mountain (2005) in which a lone drifter discovers, in the family homestead of his dead lover, the shirts they wore while cowboying together long before: shabby denim and weary cotton wrapped in each other's arms.

Director Ang Lee could not have entrusted those crucial garments to a better pair of hands. Allen's uncommon strength in supplying costumes for more than 40 productions was in telling a story through clothes. She could run off the major period frock (post-crinoline Scarlett O'Hara for a 1994 TV sequel to Gone with the Wind) along with extreme fantasy (the Incredible Hulk's expanding purple pants in 2003), but they were always grounded in and surrounded by reality.

She found her style early. Her Norwegian mother had married her English father just before the second world war. With peace, they resumed their business of smart hotel-keeping, and sent her off for holidays with Norwegian relatives, prosperous yet skilled at crafts. She told the British Library's oral history of fashion that her grandmother taught her to embroider, knit, crochet and sew, and that she helped her spiffy mother decorate the hotel for weddings and dances. At Moreton Hall prep school in Suffolk, Allen's non-uniform wear was not the approved beige kit but bright Nordic separates with red stockings. Her classmates mocked and muddied her. She didn't care.

After senior school at Adcote in Shrewsbury, she was briefly a beatnik student in Grenoble before a rendezvous in London with a gorgeously unsuitable boyfriend. Her parents disowned her, the bloke departed, and she scrabbled along as a lift attendant in the Regent Street Jaeger shop while perching in a Chelsea near-dosshouse. Allen pestered Queen magazine for work until granted an interview with the publisher Jocelyn Stevens, to which she wore her sole fashionable outfit. She swore she could get by on a fiver a week - all he would pay, as glossies expected staff to have private incomes. When she wrote copy about the gear worn and fun had by the young rich, she was paid an extra £2.

Vogue poached Allen in 1964 for a £25 weekly salary, gave her its then broad and wide education in the business and appointed her fashion editor of its Young Ideas pages. David Bailey persuaded her to model, radically different in her granny glasses and Vidal Sassoon haircut. Allen was enraptured by couture but her zone stressed softer handwrought things, photographed on location as a wordless strip cartoon.

At that time fashion and film in London were permeable, so it was obvious to ask Allen to co-ordinate clothes for Susannah York in a swinging London caper movie, Kaleidoscope (1966), which had a novel montage of shopping that remains the definition of the 1960s boutique era. After Allen's marriage to producer Sandy Lieberson, director Nicolas Roeg wanted her to dress Julie Christie for Don't Look Now (1973), an experiment in mixing vintage and fashion - "you could take things from everywhere and incorporate them into representing a woman's choices in her life", including a cashmere sweater. She had to fight for that fibre and not a cheaper substitute.

Allen slowly withdrew from magazines and dressed all the cast for Roeg on Bad Timing (1980), Eureka (1984), and The Witches (1990); and for Stephen Frears in The Hit (1984) where fine use of male tailoring was deployed to say a lot about its wearers, none of it nice. Her most elaborately costumed movies - Little Shop of Horrors (1986), White Mischief (1987), The Witches (1990) even Thunderbirds (2004) - had unglamorous elements, while Robin Williams' old biddy blouses as the housekeeper Mrs Doubtfire (1993) were based on those of Allen's schoolteachers. The only experience she was ambivalent about was Eyes Wide Shut (1999) for Stanley Kubrick. She admired him but he did not trust her instincts, and demanded the contents of costume houses just to choose a dressing gown.

After Norway, Allen's heart belonged in the American west. She had fainted with excitement as a teen when she saw Fess Parker (Disney's Davy Crockett) live on a tour - stunned more by his buckskins than his person. She researched for Jim Jarmusch's Dead Man (1995) through melodramatic photo-portraits taken in frontier boom towns. (Television's Deadwood owes her a nod for its combination of flash and squalor.) Her first work for Ang Lee, Ride With the Devil (1999), about southern renegades in the US civil war, was even less familiar, for its raiders wore military uniform only as disguise. In the core shot, they cast it aside to reveal the fancy shirts their girls had sewn for them. Allen embroidered the lead rider's shirt herself in a hurry.

She arrived for the first meeting for Brokeback with a book of Richard Avedon's portraits of the west from the 1960s to the 1980s, from which she learned its subtle dress codes. It took obscure sourcing and severe scouring to put together and break apart the apparently simple wool jackets and fake fleece linings, and keep within costs. She wardrobed La Vie en Rose (2007) on a minimal budget, skimping black dresses tiny and tight for Marion Cotillard's Edith Piaf to make the star seem more frail.

She was nominated several times for Emmy and Bafta awards but did not win; the clothes, especially those shirts, perhaps too reticent.

Her marriage to Lieberson ended in divorce. He and children Lucy, Ben and Holly survive her.

· Marit Allen, fashion editor and costume designer, born September 17 1941; died November 26 2007